Progress through Stadium Freestyle mode until you unlock NRG Stadium. This is the only stadium you'll need to gold, as getting a gold or higher medal on that map also unlocks the Monster Mutt truck and 'Woof' Achievement.
Freestyle is a poorly designed mode, as you'll never get a gold or platinum medal by driving over ramps and obstacles. Instead, the trick is to
drive up walls. Point straight at a wall, hold back on the left stick to control your angle, and hopefully you'll catch some air. Make sure to
land safely and you'll score tons of points. Keep doing this until time runs out.
I scored 11,353 on NRG Stadium and earned a Platinum. This also unlocked 'Woof!' and 'Mega Ultra Freestyler.'